1. Field of the Invention
The present invention relates to a process for the preparation of alkyl polyglycosides containing an alkyl group of 8 to 24 carbon atoms, in which saccharides are glycosylated and the products obtained are transglycosylated.
2. Description of the Invention
Alkyl polyglycosides are non-poisonous and easily degradable surface-active substances. They are, therefore, used as detergents and cleaning agents and as emulsifiers and dispersants. However, they only exhibit the desired surface properties if the alkyl groups have at least 8 carbon atoms.
Alkyl polyglycosides containing long-chain alkyl radicals can be prepared entirely or in part from renewable raw materials. Due to their interesting surfactant properties in combination with very good biodegradability, these alkyl polyglycosides are gaining increased importance. These products must, necessarily, meet high aesthetic requirements for household and cosmetic application. Thus, processes by which alkyl polyglycosides can be prepared in transparent aqueous solutions of attractive color are of great interest.
Alkyl polyglycosides are, in general, prepared from saccharides and alcohols by glycosylation and transglycosylation. In a two-step process, for example, an n-butyl glycoside is first prepared by glycosylation with n-butanol, which is then converted into the desired long-chain alkyl polyglycoside by transglycosylation with a long-chain alcohol. However, the products obtained in this process are dark-colored unless additional measures are taken.
The color can be improved according to U.S. Pat. No. 4,762,918 by catalytic hydrogenation.
When long-chain alkyl saccharides are prepared, the hydroxy polycarboxylic acids citric acid, tartaric acid and malic acid can, according to U.S. Pat. No. 4,465,828, also contribute to the improvement in color.
According to EP 0,077,167, in the reaction of alcohols with aldoses or ketoses reducing agents, such as hypophosphorous acid or sulphurous acid, can also be added. These additives are advantageous at virtually every point in the process. They improve the color of the alkyl glycosides.
Preventive measures are also known. Thus, according to EP 0,102,558, C.sub.3 - to C.sub.5 -alkyl glucosides of improved color are obtained by carrying out the glucosylation in the presence of an alkali metal salt of boric acid.
According to EP 0,165,721, the color of the products can be improved by multi-step bleaching using hydrogen peroxide and stabilized by the addition of compounds releasing sulfur dioxide.
Due to the high costs of catalysts and the difficult reaction procedure, catalytic hydrogenation is problematical. In large-scale processes, hydrogen peroxide bleaching requires the storage and handling of large amounts of peroxide. In the case of all solid and liquid color-improving agents, it is furthermore difficult to remove these agents quantitatively after reaction is complete.
In EP 0,092,876, the product, after transglycosylation and neutralization are complete, is purified by distilling off the excess long-chain alcohols having 12 to 18 carbon atoms in a thin-film evaporator. However, in this subsequent purification process, dye formation during glycosylation or transglycosylation is not prevented.
Thus, a need exists for a process for preparing alkyl polyglycosides which are light-colored in which dye formation is suppressed even during glycosylation without using auxiliary substances.